In video signal coding the problems of coding still pictures (photographic videotex) or moving pictures (videoconference, visual telephony) are generally considered separately; suitable coding algorithms, which have been already internationally standardized or whose standardization is in progress, have been developed for both types of pictures.
More particularly, for moving pictures, CCITT Study Group SGXV has defined a coding process, to be employed in synchronous networks, which is the subject matter of CCITT Recommendation H 261. The process suggested by this Group is a differential process of the so-called hybrid type, wherein at a given instant:
the difference between the original current frame and the coded frame relevant to the preceding instant is calculated; PA0 a check is made on whether a motion compensation is suitable in this difference; PA0 a two-dimensional Discrete Cosine Transform is applied to pixel blocks, possibly submitted to motion compensation; PA0 the transform coefficients are quantized by variable-step quantizers and PA0 the quantized coefficients are coded by a variable-length code. PA0 a) memorizing the quantization steps used for that group in the different frames of the still picture; PA0 b) detecting and memorizing a real quantization step which is the minimum quantization step used up to the current frame; PA0 c) comparing the real quantization step and the quantization step determined for that group in the current frame by the coding control device; PA0 d) coding the transform coefficients relevant to that group with a quantization step equal to that determined by the coding control device, if the latter step is lower than the real quantization step or, if the quantization step determined by the coding control device is greater than or equal to the real quantization step, coding of the coefficients by a particular quantization step obtained by increasing by a first predetermined quantity the real quantization step, in case of poor bit availability, and with a step obtained by reducing said minimum step by a second predetermined quantity in case of high bit availability, the coding quantization step to be used in the latter case being a unitary step if the value of the real step is less than the second predetermined quantity; and PA0 e) updating the real quantization step, whenever the coding is performed with a quantization step lower than the real quantization step. PA0 the transform has been applied to the differences between a current frame and a predicted frame; PA0 transform coefficients have been coded; and PA0 a motion compensation has been performed in the prediction.
The quantization step is periodically updated depending on the filling of a transmission buffer connected between the coder and a line for transmitting the coded signals to utilization devices. For scenes with strong changes, it is also possible to apply the transform to blocks of a current frame and not to the differences.
This process is described e.g. in the papers "Motion video coding in CCITT SGXV--The coded picture format", by R. C. Nicol and N. Mukawa, "Motion video coding in CCITT SGXV--The video source coding" by R. Plompen, Y. Hatori, W. Geuen, J. Guichard, M. Guglielmo, H. Brusewitz, and "Motion video coding in CCITT SGXV--The video multiplex and transmission coding" by M. Carr, J. Guichard, K. Matsuda, R. Plompen, J. Speidel, presented at IEEE Global Telecommunications Conference & Exhibition (Globecom '88), Holywood, Fla., U.S.A. Nov. 28th-Dec. 1st, 1988, and published on pages 992-996, 997-1004 and 1005-1010 of the Conference Proceedings (Papers 31.1, 31.2, 31.3).
Some modifications to this algorithm have been also suggested to allow its use in asynchronous networks, for which there is a growing interest since their intrinsic flexibility is particularly well suited to transmission of video signals characterized by non-uniform information flow, such as the signals relevant to video-conference and visual telephony. An example of modified algorithm has been described by U.S. in the paper "An ATM adapted video coding algorithm operating at low bit-rates" presented at the Second International Workshop on 64 kbit/s Coding of Moving Video, Hanover, 1989. In this algorithm, the coding operations are basically identical to those of the algorithm suggested by the CCITT Study Group. The transmission takes place at a variable bit rate and it is characterized by an average bit rate and a peak bit rate which are set by network controlling devices before the beginning of the transmission and are periodically monitored during transmission to adapt consequently the quantization step and possibly the space and time resolution.
In the case of videoconference and visual telephony it may be necessary to transmit not only the speaker figure (moving picture component), but also schemes or written texts (still picture component). Since the coding algorithm is of differential type, the difference between successive frames ought to be null when a still picture is being coded, and hence in a short time no more bits ought to be transmitted. In practice however, owing to the noise introduced both by the transform and quantization operations and by the camera, a difference always exists between subsequent frames of a still picture in case of both synchronous and asynchronous networks, so that the coder goes on transmitting bits without enhancing coded picture quality. Hence transmission of picture coding bits utilizes resources for transmission of other information.
No solutions to this problem appear to have been suggested hitherto. One could theoretically switch to a different coding algorithm when passing from a moving to a still scene, and to code the pictures of the latter by any standard still-picture coding algorithm. This would considerably increase complexity, since two different coding systems would be required as well as hand-controlled devices to signal a scene change and cause switching from one coding system to the other. In addition, when moving picture coding is resumed, the coding algorithm requires a new initialization, which entails a period of low quality of the coded picture.